by Susan Page, USA TODAY

by Susan Page, USA TODAY

That low-key, low-energy President Obama whose performance was widely panned after the first presidential debate two weeks ago apparently stayed back in Denver. With his ambitions for a second term on the line, the president talked faster, pushed back harder and challenged Mitt Romney with more specifics in a crackling second encounter on Long Island on Tuesday.

The Republican nominee often was put on the defensive, forced to explain promises he made during the Republican primaries on immigration and taxes. He seemed taken aback when Obama blasted him for what the president characterized as an effort to score political points from violence in the Arab world that cost a U.S. ambassador's life. And in one of several confrontations that put the two men virtually toe-to-toe, Romney demanded to know whether Obama realized he had investments in China in his pension.

"Mr. President, have you looked at your pension?" Romney asked. "Have you looked at your pension? Mr. President, have you looked at your pension?"

"You know, I don't look at my pension. It's not as big as yours, so it doesn't take as long," Obama finally said to laughter.

Romney struck a regretful tone when he expressed disappointment in the president's economic tenure, a sentiment mentioned even by a voter who posed a question in the debate. "If you elect President Obama, you know what you're going to get," Romney said. "You'll get a repeat of the last four years." He called Obama "great as a speaker" but, in effect, a failure as a president in delivering prosperity for the middle class.

A few moments on TV more than likely will determine who is elected president in 21 days. The question will be which moments: the most memorable exchanges from the three 90-minute debates between Obama and Romney? Or the impact from a flood of 30-second ads both sides are airing?

Strategists in both campaigns viewed the second debate as a pivotal event in a contest that is once again strikingly close. After a year-long campaign, the town-hall-style forum at Hofstra University was probably the best remaining opportunity for Romney to build on the positive impression he made in the first debate and for Obama to regain the momentum he lost there.

Obama came across as more engaged and presidential than he did in Denver, and he hit themes he failed to spotlight in the first debate. In response to the first question, he managed to mention Romney's opposition to the auto bailout. In the last, he noted Romney's controversial "47%" comment. And in remarks that seemed tailor-made to please female voters, he noted his debt to his single mother and his grandmother and his aspirations for his two daughters.

The confrontation between the two men at times seemed almost physical as they circled the stage, addressed one another directly, all but accused each other of lying and complained to moderator Candy Crowley of CNN that fairness meant he deserved more time to speak. There were flashes of heat.

Given the buildup beforehand for the high-stakes encounter, the debate presumably drew a huge audience. The first was watched by 67 million people, Nielsen reported, the most for any presidential debate since 1992.

Still, in the battleground states, the debates are framed by an unprecedented onslaught of political advertising, almost all of it negative. On local stations from Orlando to Columbus, Richmond to Las Vegas, airtime from now until election eve is sold out and campaign ads are running back-to-back.

It is easier to skip the debates than avoid the spots. In a USA TODAY/Gallup Poll of Swing States, nearly eight of 10 residents in a dozen top battlegrounds say they have seen an ad for Obama or Romney in the past three days. (So did 63% of those in non-swing states.)

For most, it wasn't a positive experience. A 55% majority in the battlegrounds say this year's presidential ads have "turned them off to the campaign." Just a third say they have made them "feel more excited" about it.

"It's just mud-slinging," Mary Edwards, 46, of Wilson, N.C., who was called in the poll, said in a follow-up phone interview. "They're just talking trash about each other. They're not putting out what they're going to do, what they can do."

That said, voters in the battlegrounds acknowledge that the ads are influential. Six in 10 say the campaign ads they've seen have made them more certain about whom they'll support. Fewer than one in five say the ads made them less certain.

The poll of 1,023 registered voters was taken Oct. 5-11 in Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, New Mexico, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin. The margin of error is +/-4 percentage points.

In this campaign more than previous ones, the ads and the debates seem to be filling divergent roles. Almost all of the TV ads have been negative - that is, devoted to blasting the other candidate. A study by the Wesleyan Media Project released earlier this month reported that fewer than 8% of the presidential ads in the previous three weeks had been positive, mentioning only the candidate it backed. That's much lower than in 2000, when 30% of the ads were positive, or in 2008, when 19% were.

In some ways, the debates are designed to be an antidote to ads. On a small stage, surrounded on three sides by an audience of about 80 undecided or "persuadable" voters, they tried to make the case for themselves. Obama touted his record as president; Romney bragged about his record as Massachusetts governor and head of the Salt Lake City Olympics.

But they also turned nearly every question to an attack on the other. In an emotional exchange, Romney accused Obama of behaving inappropriately by leaving Washington to headline political fundraisers on the day after the U.S. ambassador was assassinated in Libya, and of trying to mislead Americans about the terrorist nature of that attack.

Obama turned the charge back to Romney: "The suggestion that anybody on my team, the secretary of State, our U.N. ambassador, anybody on my team, would play politics or mislead when we've lost four of our own is offensive, governor."

Their exchanges weren't exactly spontaneous - each candidate spent days at "debate camps" beforehand rehearsing quips and practicing answers - but they are less scripted than almost anything else the candidates do during a campaign. "An unfiltered moment," Republican National Chairman Reince Priebus said beforehand.

But in the end, says Mitchell McKinney of the University of Missouri, "the power of the 30-second spot may well trump the effect of debates."

For one thing, he says, voters are more likely to recall ads, especially negative ones, than they are moments from a debate beyond some gaffe or exchange that becomes the subject of constant replay. For another, ads reach the "marginally attentive voter" while debates tend to attract voters who already have made up their minds and are rooting for a candidate. Ads can be targeted to specific voters in particular states; the message in a nationally televised debate is broader.

Debates can help maintain or change momentum, as the first debate boosted Romney, and as the second debate may revive Obama's prospects. But after next week's final debate in Florida, focused on foreign policy, the candidates and their allies will intensify their campaigns on the airwaves. Romney raised $170 million last month and Obama $181 million, both records for this campaign cycle, with appeals to supporters to help pay for even more of those ads.